Electronic devices typically require a connected (wired) power source to operate, for example, battery power or a wired connection to a direct current (“DC”) or alternating current (“AC”) power source. Similarly, rechargeable battery-powered electronic devices are charged using a wired power-supply that connects the electronic device to a DC or AC power source. The limitation of these devices is the need to directly connect the device to a power source using wires.
Wireless power transfer (WPT) involves the use of time-varying magnetic fields to wirelessly transfer power from a source to a device. Faraday's law of magnetic induction provides that if a time-varying current is applied to one coil (e.g., a transmitter coil) a voltage will be induced in a nearby second coil (e.g., a receiver coil). The voltage induced in the receiver coil can then be rectified and filtered to generate a stable DC voltage for powering an electronic device or charging a battery. The receiver coil and associated circuitry for generating a DC voltage can be connected to or included within the electronic device itself such as a smartphone.
The Wireless Power Consortium (WPC) was established in 2008 to develop the Qi inductive power standard for charging and powering electronic devices. Powermat is another well-known standard for WPT developed by the Power Matters Alliance (PMA). The Qi and Powermat near-field standards operate in the frequency band of 100-400 kHz. The problem with near-field WPT technology is that typically only 5 Watts of power can be transferred over the short distance of 2 to 5 millimeters between a power source and an electronic device, though there are ongoing efforts to increase the power. For example, some concurrently developing standards achieve this by operating at much higher frequencies, such as 6.78 MHz or 13.56 MHz. Though they are called magnetic resonance methods instead of magnetic induction, they are based on the same underlying physics of magnetic induction. There also have been some market consolidation efforts to unite into larger organizations, such as the AirFuel Alliance consisting of PMA and the Rezence standard from the Alliance For Wireless Power (A4WP), but the technical aspects have remained largely unchanged.
Some techniques for WPT use two or more transmitter coils in an attempt to overcome the issue of low power transfer over short distances. Typically, two identical transmitter coils (e.g., both wound in the clockwise direction or both wound in the counter-clockwise direction and having substantially the same shape, equal number of turns and substantially the same area) are coupled in series or parallel on a single magnetic layer to transfer power to a receiver coil. Alternatively, the coils can be placed in close proximity to one another without the use of a magnetic layer. This configuration results in the applied time-varying current flowing through both coils in the same direction at any point in time, generating an almost perpendicular combined magnetic field with flux lines that flow from both coils in the same direction (i.e., the magnetic field generated by either coil has the same polarity as the other coil). Magnetic flux lines tend to repel if they are in the same direction, which causes the flux lines to radiate through the air for great distances, increasing the likelihood of failing Electromagnetic Interference (EMI) requirements. EMI requirements, including, for example, spurious emissions requirements, are set forth by the Federal Communications Commission (FCC) and the European Telecommunications Standard Institute (ETSI). When magnetic flux lines radiate away from a transmitter coil into the environment, there is a higher likelihood that energy will be emitted in neighboring frequency bands that will exceed spurious emission requirements.
When magnetic flux lines repel, the magnetic reluctance is high, resulting in a weak magnetic field that reduces the amount of magnetic coupling between the transmitter coils and a receiver coil placed in close proximity (i.e., 2-5 millimeters) to the transmitter coils. So although the coil area is larger than in a single-coil transmitter, the resulting magnetic flux available to transfer power is reduced. If the transmitter coils are placed on separate magnetic layers, an air gap exists between the magnetic layers resulting in an even weaker generated magnetic field as the air gap further increases the reluctance between the transmitter coils.
Transmitters that comply with existing WPT standards also achieve substantially lower power transfer to an electronic device if the receiver coil in the electronic device is not properly centered over the transmitter coil. One technique used to address this issue is increasing the frequency of the time-varying current applied to the transmitter coil (e.g., frequencies above 400 kHz). Although higher frequencies of operation may increase the amount of power transferred over the distance between a transmitter coil and a receiver coil, higher frequencies of operation may interfere with the operation of other devices and may also fail EMI requirements, including, for example, spurious emissions by radiating energy into neighboring frequencies.
Due to the short range of existing WPT technology, for effective power transfer the receiver coil connected to a device must be centered with the transmitter coil connected to a device and the coils cannot be more than 2-5 millimeters apart. One technique for centering a receiver coil in an electronic device with a transmitter coil in a wireless power transmitter is via magnets. However, this technique is not capable of ensuring perfect alignment between the coils as the magnets could be offset from one another and the user would not otherwise be aware of the offset. Further, the use of magnets is practical only for devices that are perfectly flat and devices that are large enough to accommodate large magnets that maintain a strong connection between the transmitter and receiver devices. Even where the use of magnets is possible, magnets are known to cause eddy current losses when used in conjunction with coils, which degrade overall system efficiency. For this reason, the Qi standard has started phasing out previously approved coils with built-in magnets.
Another technique for aligning a receiver coil with a transmitter coil involves the addition of low-power detection coils for each axis of the electronic device. For example, one detection coil can be used for measuring x-axis offset and a second detection coil can be used for measuring y-axis offset. When a single coil is used to measure offset for each axis of an electronic device, each coil is aligned such that the axis being measured passes through the center of the coil, resulting in the coil being positioned perpendicular to the axis. That means both the x-axis and y-axis coils are positioned parallel to the magnetic field generated by the transmitter coil, resulting in a very small surface area for coupling of the magnetic field. Accordingly, each coil receives only a small percentage of the magnetic field generated by the transmitter coil, which results in a proportionately small induced voltage in each detector coil on the order of micro-volts. Each low-power detection coil is connected to a high impedance voltage detector, which typically includes an amplifier stage. Due to the small induced voltage, the voltage must be amplified to calculate an offset associated with any misalignment of a given receiver coil with the transmitter coil. The receiver device will then emit a sound when the calculation based on the detected induced voltage suggests that that the receiver and transmitter coils are centered. One problem with this technique is that the induced voltage, which is on the order of micro-volts, is susceptible to noise interference resulting in substantial error in the offset calculation. Although the receiver device may emit a sound indicating to the user that the transmitter and receiver coils are aligned, the coils may actually be offset due to error in the offset calculation caused by noise. The offset error is further magnified by the need to amplify the induced voltage. Another problem with this technique is that it is not possible to determine the direction of offset when using one detection coil per axis. The offset calculation simply provides a magnitude (e.g., ½ inch) and is not capable of specifying whether the offset is ½ inch to the right or ½ inch to the left, requiring significant trial and error for the user to manually find the calculated alignment between the transmitter and receiver coils. Because a misalignment between a receiver coil and a transmitter coil can result in significant reduction in the amount of power transferred, the current state of WPT technology is not suitable for many consumer devices.